1. Field of the Invention
The invention relates to a technology that restrains the meandering of a sheet-like object that is conveyed lengthwise along the conveying path.
2. Description of the Related Art
As a production method for a sheet electrode that is a raw material of positive electrodes of lithium-ion secondary cells, there is known a method in which an active material layer is formed by applying a predetermined active material to a sheet surface of a sheet made of aluminum (hereinafter, referred to as “aluminum sheet”), and the density of the active material layer is raised by pressing the aluminum sheet with the active material layer formed thereon, by rollers or the like.
Generally, a production apparatus for such a sheet electrode includes an application device that applies a predetermined active material to a sheet surface of an aluminum sheet, a desiccation device that dries the active material by heating the aluminum sheet on which the predetermined active material has been applied, a press device made up of press rollers which presses the aluminum sheet on which the active material has been applied and dried, and a conveyance device that connects the application device, the desiccation device and the press device and conveys the aluminum sheet. As the conveyance device, there is known a device which includes a driving roller that contacts an aluminum sheet and is rotationally driven, and a driven roller that rotatably contacts the aluminum sheet, and which conveys the aluminum sheet in its longitudinal direction.
Generally, a positive electrode of a lithium-ion secondary cell is produced by applying a predetermined active material to a central portion of a sheet surface of an aluminum sheet that is central in the width direction of the sheet, and forming a “region to which the active material is not applied” in each of two end portions of the sheet surface in the width direction, and then cutting the aluminum sheet at the center thereof in the width direction. During production of the lithium-ion secondary cell, a wiring for electrification is welded to the “region to which the active material is not applied”. In order to secure a certain quality of the lithium-ion secondary cells that employ positive electrodes produced as described above, it is necessary to accurately control the width of the “region to which the active material is not applied” formed on a sheet surface of an aluminum sheet that is to be made into positive electrodes. Then, in order to accurately control the width of the “region to which the active material is not applied”, the deviation of the aluminum sheet during the conveyance thereof by a conveyance device, the meandering of the aluminum sheet, must be restrained.
Besides, from the viewpoint of improving the production efficiency of lithium-ion secondary cells and thus cutting back the production cost of lithium-ion secondary cells, it is important to raise the velocity at which a conveyance device conveys the aluminum sheet.
However, in general, the conveying velocity of a sheet-like object, such as the foregoing aluminum sheet or the like, and the meandering of the sheet-like object are in a trade-off relation; that is, if the conveying velocity of the sheet-like object rises, the meandering of the sheet-like object tends to become conspicuous.
A known method for restraining the meandering of a sheet-like object is a method that uses a meander control device that includes a guide roll that is provided on an intermediate portion of a conveying path of a sheet-like object, and that contacts the sheet-like object, and alters (corrects) the conveying direction of the sheet-like object by turning about an axis perpendicular to the sheet surface of the sheet-like object conveyed along the conveying path, an edge sensor that is disposed on the conveying path at a downstream side of the guide roll, and that detects the position of the sheet-like object in the width direction and therefore the amount of meandering of the sheet-like object, and a control device that feedback-controls the amount of turn (angle of turn) of the guide roll on the basis of the amount of meandering of the sheet-like object that is detected by the edge sensor.
However, in the foregoing method of feedback-controlling the amount of turn of the guide roll, since the edge sensor is disposed at the downstream side of the guide roll, and corrects the meandering of the sheet-like object subsequently to occurrence thereof, the method leads to an event as follows. That is, particularly in the case where the amount of meandering of the sheet-like object sharply increases or decreases under a condition that the conveying velocity of the sheet-like object is great, if the gain of the control device is made large, the meandering of the sheet-like object is rather promoted; on the other hand, if the gain of the control device is made small, the meandering of the sheet-like object (deviation of the sheet-like object from a position through which the sheet-like object is required to pass in the conveying path) cannot be eliminated. Thus, in the case where the conveying velocity of the sheet-like object is great, there arises a problem of the meandering of the sheet-like object being unable to be restrained merely by feedback-controlling the amount of turn of the guide roll.
A known method for solving this problem of the feedback control is a method in which an edge sensor is disposed at an upstream side of the guide roll along the conveying path, and the amount of turn of the guide roll is feedforward-controlled on the basis of the amount of meandering of the sheet-like object detected by the edge sensor. An example of this method is described in Japanese Patent Application Publication No. 5-39527 (JP-A-5-39527).
Generally, the method of feedforward-controlling the amount of turn of the guide roll calculates an estimated value of the amount of meandering of the sheet-like object at a position of contact with the guide roll (guide position) (hereinafter, the estimated value will be referred to as “guide position-estimated amount of meandering”), and controls the amount of turn of the guide roll so as to cancel out the calculated “guide position-estimated amount of meandering”.
However, the foregoing method of feedforward-controlling the amount of turn of the guide roll has a construction in which coefficients, constants and other parameters used in the calculation expression for calculating the “guide position-estimated amount of meandering” are set beforehand as fixed values on the basis of experiments or theoretical values. Therefore, in the case where the conveying condition of the sheet-like object changes due to an external disturbance factor, and the feedforward control is performed in a conveying condition that has deviated from the conveying condition set at the time of setting the foregoing parameters, the accuracy of the calculated “guide position-estimated amount of meandering” declines.
An example of the foregoing external disturbance factor is a change of the friction force (friction coefficient) between a group of rollers that constitute the conveyance device and the sheet surface of the sheet-like object. The friction force between the group of rollers constituting the conveyance device and the sheet surface of the sheet-like object can change in various cases, including: (1) the case where the conveying velocity or tension of the sheet-like object changes, for example, when the conveyance device starts conveying the sheet-like object, or stops conveying; (2) the case where the tension of the sheet-like object changes because of, for example, malfunction of the dancer rolls that control the tension, or the like; (3) the case where the surface roughness of surfaces of rollers constituting the conveyance device changes because of, for example, abrasion of the surfaces of the rollers, or the like; (4) the case where the thickness of the sheet-like object being conveyed, or the length thereof in the width direction or the longitudinal direction changes (e.g., in the case of a positive electrode of a lithium-ion secondary cell, the effective thickness thereof increases when an active material is applied to the sheet surface. Or, when the sheet-like object is heated and cooled in order to dry the active material applied thereon, the sheet-like object expands and shrinks, thus changing its length in the longitudinal direction); (5) the case where there are variations in the surface roughness or thickness of sheet-like objects between lots; etc. Thus, in order to effectively restrain the meandering of the sheet-like object by the related-art feedforward control, there is a need to perform, as a prerequisite condition, a strict process management so as to exclude the foregoing external disturbance factor, thus giving rise to a problem of it being difficult to apply this requirement to actual operation.